The demand for services in which data is delivered via a wireless connection has grown in recent years and is expected to continue to grow. Included are applications in which data is delivered via cellular mobile telephony or other mobile telephony, personal communications systems (PCS) and digital or high definition television (HDTV). Though the demand for these services is growing, the channel bandwidth over which the data may be delivered is limited. Therefore, it is desirable to deliver data at high speeds over this limited bandwidth in an efficient, as well as cost effective, manner.
A known approach for efficiently delivering high speed data over a channel is by using Orthogonal Frequency Division Multiplexing (OFDM). The high-speed data signals are divided into tens or hundreds of lower speed signals that are transmitted in parallel over respective frequencies within a radio frequency (RF) signal that are known as sub-carrier frequencies (“sub-carriers”). The frequency spectra of the sub-carriers overlap so that the spacing between them is minimized. The sub-carriers are also orthogonal to each other so that they are statistically independent and do not create crosstalk or otherwise interfere with each other. As a result, the channel bandwidth is used much more efficiently than in conventional single carrier transmission schemes such as AM/FM (amplitude or frequency modulation).
Another approach to providing more efficient use of the channel bandwidth is to transmit the data using a base station having multiple antennas and then receive the transmitted data using a remote station having multiple receiving antennas, referred to as Multiple Input-Multiple Output (MIMO). The data may be transmitted such that there is spatial diversity between the signals transmitted by the respective antennas, thereby increasing the data capacity by increasing the number of antennas. Alternatively, the data is transmitted such that there is temporal diversity between the signals transmitted by the respective antennas, thereby reducing signal fading.
In OFDM and MIMO systems, a preamble may be inserted within a signal frame in order to provide: base station identification and selection, CIR measurement, framing and timing synchronization, frequency synchronization as well as channel estimation. In many cases, the preamble search requires a large amount of computation power at the subscriber station. For the initial cell search, there is no prior knowledge about the synchronization positions for potential base station candidates; hence the subscriber station needs to perform the correlations with all possible pseudo noise (PN) sequences for each Fourier fast transform window position within the entire searching window. Such a window could be large even for the synchronous bases station network. For handoff, even with the presence of the adjacent base station list information broadcast from the anchoring base station, the preamble search is of excessively high computational complexity.
Advancements to communication systems such as those standardized in the evolution of WiMAX have resulted in concepts that build upon the initial frame structure found in the original 802.16e standard. These concepts result in new possibilities for addressing and synchronizing devices within the communication system. These concepts and possibilities also may be applied to any 3GPP or 3GPP2 system.
It is therefore desirable to provide preambles that enable easy, fast synchronization between the subscriber station and the base stations and that provide low complexity and fast cell search after coarse synchronization.
Accordingly, there is a need for an improved preamble design, method and apparatus which are suitable for the mobile, broadband wireless access systems.